Electrowetting display devices are known for example from PCT publication no. WO/2003/071346. Hysteresis may be observed in electrowetting display devices. Hysteresis causes the electrowetting display to behave differently with an increase of applied voltage, compared with a decreasing voltage. Consequently, a display effect provided by the display may be inconsistent at a given voltage, depending on whether the applied voltage is reached from a lower or higher previous voltage. This is problematic for a display requiring reliable display states.
The article “Gray Scales for Video Applications on Electrowetting Displays”, by R. van Dijk, B. J. Feenstra, R. A. Hayes, I. G. J. Camps, R. G. H. Boom, M. M. H Wagemans, A. Giraldo, B. v.d. Heijden, R. Los and H. Feil, Liquavista, ISSN0006-0966X/06/3701-0000 describes pulse width modulation driving of an electrowetting pixel to only two extreme values; a pixel on value and a pixel off value. Increasing the pulse width, i.e. increasing the pulse duration, increases the brightness of the pixel. Various gray scales can therefore be obtained by changing the pulse width. By driving the pixel to only the two extreme levels, hysteresis has no influence on the optical performance.
The article “An Accurate Gray-level Driving Scheme for a Large-area High-resolution Electrowetting Display” by Yi-Cheng Chen, Chao-Chiun Liang, Yung-Hsiang Chiu, Wei-Yuan Cheng, Kuo-Lung Lo and Yu-Pei Chang, Display Technology Center (DTC)/Industrial Technology Research Institute (ITRI), 94, IDRC 08, ISSN1083-1312/00/2008-0094 describes using alternating current (AC) modulation to deal with hysteresis. Using AC requires twice the operating voltage than a direct current (DC) scheme; power consumption and operating costs are therefore higher, which is undesirable.
It is noted that PCT patent publication no. WO/2005/036517 addresses the reproducibility of grayscales in electrowetting displays using AC pre-pulses before each greyscale state. PCT patent publication no. WO/2007/057797 describes a voltage signal, for example of zero volts, which is different from a data write signal and is used for preventing oil backflow. Neither of these disclosures relate to the problem of hysteresis described above.
An object of the present invention is to overcome problems caused by hysteresis.